Such a burner is the subject matter of a Patent Application No. FR-2 656 676 published on 5 Jul. 1991 and filed jointly by IFP (INSTITUT FRANCAIS DU PETROLE) and the firm PILLARD EGCI: the burner described in this Patent Application effectively makes it possible to reduce the formation of nitrogen oxides (NOX), thanks in particular and in combination with a "rose" stabilizer with blades about a central hub, to the creation of a plurality of independent flames: these latter are described as one of the characteristics of the invention, although multi-flame burners were already known, such as in Application FR 2 503 836 published on 15 Oct. 1982, where a stepped combustion is effected, as a quantity of air passes between the adjacent jets of fuel and thus penetrates further in the zone of combustion before meeting the fuel.
However, the difficulty inherent in this arrangement consists in maintaining a combustion of good quality, i.e. without non-burned residues, as, if the effect of stepping allowing the reduction of nitrogen oxide is accentuated with a reduced number of elementary flames, the final mixture of the air and fuel is more difficult and a part of the air may attain the outlet of the hearth without having participated in combustion, in that case creating non-burned residues; to avoid this, it is possible to increase the excess air supplying the burner, but in that case there are other drawbacks, such as the reduction in output and the increase of the free oxygen in the hearth, which causes the formation of nitrogen oxide to rise and is therefore contrary to the purpose aimed at.
Another process of stepping the air is also known, used in particular in boilers of thermal stations, operating essentially with pulverized coal since the 1970's, and which consist in sending only a part of the combustion air into the burner so as to create a primary combustion zone with excess of fuel, therefore with a low free oxygen content and a slight formation of nitrogen oxide; the remains of the air are introduced in the hearth in the form of so-called secondary air, either via an annular ring around the burner, or via the orifices made in the walls of the hearth at a more or less great distance from the burner; this complementary air is supposed to create a secondary combustion zone making it possible to burn all the fuel.
Such a process and device are described in Patent Application No. FR 2 450 998 of the firm ST. EINMULLER GmbH, filed under German priority and published on 3 Oct. 1980, entitled "Process for reducing the emission of NOX at the outlet of a burner".
The drawbacks of such a process are also the difficulty in obtaining a good air and fuel mixture in the so-called secondary zone, which has for a consequence considerably to increase the length of the flames and to increase the quantity of non-burned residues.
This Patent Application No. FR 2 450 998 specifies the reactional mechanisms provoking the formation of nitrogen oxide in industrial furnaces and of which the reduction of the emission is the object of the two Patent Applications mentioned previously, and of those cited hereinafter, as well as of the present invention.
In fact, it is recalled that this nitrogen oxide is due essentially to two different origins, namely:
the formation of NOX based on the oxidation of nitrogen in the combustion air itself, which can only be effected with the existence of atomic oxygen or other aggressive radicals, such as OH or O.sub.3, as well as with a very high temperature in the combustion chamber, hence a first notion of thermic NOX, PA1 the formation of NOX from the oxidation of nitrogenous compounds existing in the fuel; during pyrolysis within the flame, there are formed from these compounds nitrogen and carbon or nitrogen and hydrogen radicals, which are oxidized into NOX in the presence of oxygen, due to the reactivity to this gas, even at relatively low temperatures, hence a second notion of fuel NOX. PA1 Application EP 377 233 published on 11 Jul. 1990, filed by the Dutch firm REMEHA FABRIEKEN and entitled "Atmospheric gas burner with low NOX rate", and comprising flat elements extending parallel on the two sides of said flames of the burners; PA1 or European Application EP 280 568 published on 31 Aug. 1988 and filed by the firm BABCOCK-HITASHI KABUSHIKI KAISHA under Japanese priority and entitled "Combustion apparatus with low NOX concentration", and comprising pipes for distributing pulverized coal, secondary air and tertiary air, as well as gas-distributing pipes disposed in mobile manner. PA1 said fuel is injected in several divergent directions in the hearth to create independent flames therein; PA1 said secondary air is injected by as many injectors as there are independent flames, each of said secondary air injections being effected axially and angularly with respect to each of the flames depending on a position of said injectors, such that the additional air flux is brought after a first combustion phase.
Taking this double possibility of formation into account, it is known to be necessary, on the one hand, to reduce in the flames the content of free oxygen which risks combining with the nitrogen of the fuel and, on the other hand, to step this combustion in order to reduce the peak temperatures and to increase the rate of burned gas recycled in the flame.
As a function of these considerations, and in addition to the two Patent Applications mentioned hereinabove and describing two types of solutions for attaining this object, other manufacturers have developed particular techniques, of which certain have also formed the subject matter of Patent Applications, such as:
Other Applications may also be mentioned, each relating to a specific type of fuel and different particular devices for injection and combustion of said fuel, in order to obtain the object sought after; this latter is effectively attained in the majority of cases, but, with limits since, as indicated hereinabove, when the rate of nitrogen oxide is reduced by one of the means described in the Patent Applications mentioned above, too much, a drop in output is often created, and poor combustion, in that case creating non-burned residues. Moreover, each fuel has inherent combustion characteristics and the devices most adapted to one type of fuel generally cannot be transposed on another, and sometimes even the contrary.
The problem raised is therefore that of being able to produce a burner essentially with fluid, i.e. liquid or gaseous, fuel, from any known, for example axial supply burner device, but of which it is desired to reduce to a very low level the emission of nitrogen oxide, without, however, reducing the power thereof, nor increasing the non-burned exhaust gases.
One solution to the problem raised is a process for exploiting a known fluid fuel burner comprising means for injecting the fuel into a hearth , at least one conduit for supplying primary air and a flame stabilizer around said injection means and at least one conduit for supplying secondary air located radially on the periphery outside the primary air supply conduit, in which: